Method for sealing a metal cans with peelable lids and device therefor

ABSTRACT

A method of sealing a peelable lid ( 2 ) to a flange ( 4 ) provided within a metal can body ( 1 ) which involves inserting an induction coil ( 6 ) into the can body to primarily heat the sealing surface ( 5 ) of the flange, while keeping the exterior wall of the can relatively cool to avoid tin reflow and decoration degradation of the exterior wall. The induction coil is subsequently removed from the can body and a peelable lid applied to the flange, whereby residual heat in the flange aids the sealing of the lid to the flange, e.g. by allowing activation of a bonding material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 14/358,275, now U.S. Pat. No. 10,040,588, which is the NationalStage of International Application No. PCT/EP2012/069859, filed Oct. 8,2012, which claims the benefit of EP application number 11190398.5,filed Nov. 23, 2011, the disclosures of which are incorporated herein byreference in their entirety.

TECHNICAL FIELD

The present invention relates to metal cans with peelable lids and moreparticularly to a method of sealing peelable lids to metal cans usinginduction heating.

BACKGROUND

Many containers used to hold food products have a peelable lid which iseasily detached prior to first use of the product by a consumer. Suchpeelable lids act both to seal the dispensing aperture of the containeras well as to provide a tamper evidence indicator. A peelable lidprovides an easy method of opening a container without the need forseparate tools, such as scissors or can openers.

Containers with peelable lids can be made from a variety of differentmaterials including glass, cardboard, plastic and metal. Cardboardcontainers may be suitable for holding food products in some situations;however, in certain markets, for example countries with relatively hotand/or humid climates, cardboard containers may not be appropriate.Susceptibility to attack by pests (e.g. mice and rats) and theirrelative lack of strength may also make cardboard containers unsuitablefor many uses. Plastic and glass containers are also prone to some ofthe same problems. Metal containers or “cans” address many of theseissues.

Cans for use in specialised market segments, for example cans used inthe infant formula market, are required to satisfy stringent safetystandards. This can be a challenge when producing a metal can with apeelable, typically foil, lid. For example, it is required that a canused to hold infant formula powder maintain its hermetic seal with apeelable lid even when stored in excess of 3 months at hightemperatures, such as 45° C., and with a pressure difference of 700 mbar(70 kPa) between the inside and outside of the can.

Typically, a hermetic seal between a peelable foil lid and anappropriately configured inwardly directed flange or lip of a metal canis made by first heating the sealing surface of the flange or lip.Either the sealing surface or an opposed sealing surface of the lid iscoated with a bonding material, typically either a lacquer or a polymer.

The foil lid is applied to the flange and a seal is achieved by applyinga combination of heat and pressure (applied to the lid in situ).

Where the filled, sealed can is to be subsequently processed to cook orotherwise heat the contents, a polypropylene coating may be provided onthe interior surface of the can and on the lidding material such thatthese “weld together” to form a hermetic seal. Seals provided in thisway are able to withstand the rigours of processing, such as acombination of high temperature (typically 120° C. or more) and pressureacting on the lid.

Heating of the flange may be achieved using conduction heating orinduction heating. In the case of conduction heating, heat istransferred to the metal can via direct contact with the can. In thecase of induction heating, a high frequency alternating current ispassed through an electromagnetic induction coil to produce anelectromagnetic field. The coil is placed around the outside of the cansuch that the can, and in particular the area surrounding the flange, issituated within the axially induced electromagnetic field. The resultingeddy currents created in the flange and surrounding can area give riseto a rapid heating of the flange. Induction heating is generallyadvantageous as compared with conduction heating as heating times withthe former are faster, and direct contact with the can is not required(such that various can shapes can be used with the same inductionheating set up).

For some uses, it is desirable to provide the foil lid at a positionpart-way along the can body, thus separating the can body into twocompartments. The (hermetically sealed) compartment beneath the foil lidis used to contain the food product, whilst the compartment above thelid may contain some other item, e.g. a plastic spoon. The top openingof the can may be closed with a plastic lid or the like. Particularlyfor cans of this configuration, the known heating schemes can causenoticeable damage to the exterior of the can due to the hightemperatures that the exterior must be heated to, in order to raise theflange sealing surface to the temperature required to achieve a seal.Typically, as the flange will cool slightly between initial heating andapplication of the foil lid, the sealing surface must be heatedsignificantly above the setting temperature. In addition, as heat isconducted from the exterior surface of the can to the flange, thatexterior surface must in turn be heated to an excessive temperature. Fora bonding material that operates at around 160° C., the flange needs tobe heated to 200° C., during which the exterior surface can reach atemperature of 280° C. The temperature required at the exterior surfaceis such that tin reflow may occur, resulting in visible marking (i.e.discoloration) to the exterior surface.

One possible solution is to use a bonding material which operates at alower temperature than those that have typically been used for thispurpose, e.g. a bonding material that sets at 90° C., so that it is notnecessary to excessively heat the exterior surface of the can. However,this would render the cans unsuitable for sale in markets with hotclimates.

SUMMARY OF THE INVENTION

It is an object of the invention to overcome or at least mitigate thedisadvantages of known induction heating systems of the type used insealing a peelable lid within a can body.

According to a first aspect of the present invention there is provided amethod of sealing a peelable lid to an inwardly projecting andcircumferentially extending flange of a metal can body. The methodcomprises inserting an induction coil into the can body and passing analternating current through the coil to heat the flange. The inductioncoil is then removed from the can body and a peelable lid applied to theflange, whereby residual heat in the flange seals or aids the sealing ofthe peelable lid to the flange.

An advantage of at least certain embodiments of the invention is that,whilst the flange can be heated to the required temperature, theexterior wall of the can is not “overheated” thereby preventing tinreflow at, and degradation of, the external surface.

The method may involve locating a further induction coil around theexterior of the can body and, substantially simultaneously with the stepof passing an alternating current through the inner coil, passing analternating current through the further coil to apply supplementary heatto the flange. The first and second coils may be energised together orseparately.

In some embodiments, a bonding material may be applied between thepeelable lid and the flange of the can. In other embodiments, a bondingmaterial may be applied to the sealing surface of the flange prior toapplying the peelable lid to the flange. In still further embodiments, abonding material may be applied to the sealing surface of the peelablelid prior to applying the lid to the flange.

In some embodiments, the flange is located part way along the length ofthe can body such that sealing of the peelable lid to the flangeseparates the can body into upper and lower compartments.

According to a second aspect of the present invention there is provideda heating system for use in a can production line and comprising aninduction coil for insertion into a can body and a power supply forenergising the coil whilst it is within the can body.

The heating system may comprise a further coil for locating around theexterior of the can body, said power supply being configured to energisethe further coil when it is around the can body. The first mentionedcoil and said further coil may be mechanically coupled together toprovide a single operating unit, i.e. such that a can body can be movedrelative to the unit so as to move the coils into position with respectto the can body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically a metal can separated into twocompartments by a peelable foil lid including, as inset, a detailshowing a flange, sealing surface of the flange, bonding material andthe peelable lid;

FIG. 2 is a perspective view of a metal can located within an inductioncoil for the purpose of heating a sealing flange;

FIG. 3 is a perspective view of a flange heating system comprising aninduction coil located within a metal can;

FIG. 4 is a perspective view of an alternative flange heating systemcomprising two induction coils, positioned within and outside of thecan;

FIG. 5 is a perspective view of a further alternative flange heatingsystem; and

FIG. 6 illustrates schematically a production line used to hermeticallyseal foil lids to metal can bodies.

DETAILED DESCRIPTION

Sealed peelable lids provide consumers with an easy method of opening acontainer, providing both convenience and safety. Moreover, asillustrated in FIG. 1, peelable lids 2 can be used to separate a metalcan body 1 into two separate compartments, where the can body 1 itselfmay be formed by folding a flat sheet and providing an axial weld or bypunching a circular disc to form a cylinder with an integral base. Theinset in FIG. 1 shows in detail a cross-section of the region where thelid seals to an upper sealing surface 5 of an inwardly projecting flange4, by means of a bonding material 3. The can itself is typically made oftinplate, with the flange 4 being formed by pressing in a circulargroove around the circumference of the can 1 and subsequently applyingan axial compression force to the can 1 to collapse the groove. The lid2 is typically formed of a metal foil or of a plastic or paper material.

As discussed previously, when sealing the lid 2 to the flange 4 of thecan body 1, heat may be applied to the flange 4 using induction heating.Conventional approaches to induction heating, such as that illustratedin FIG. 2, which uses a single external coil 6, can however result intin reflow on the exterior surface of the can or other effects thatcause visible surface degradation. It is desirable to provide a methodof sealing a peelable lid 2 to a flange 4 of a can body 1 which directsheat to the flange 4 whilst reducing the extent to which the exteriorsurface of the can is heated.

This is achieved using a flange heating system as illustrated in FIG. 3.The flange heating system comprises an induction coil 7. A can 1 israised and lowered with respect to the flange heating system such that,during heating, the induction coil 7 is inserted within the metal canbody 1, adjacent to the flange 4, and then removed from the can 1 afterheating. Following insertion into the can, the gap between the coil andthe flange is relatively small, e.g. on the order of 1 mm. Thistolerance is sufficient to allow the coil to be moved into and out ofthe can at the high speeds necessary on a production line.

Contrary to established understanding and practise, it has been foundthat a coil inserted within a can 1, rather than around the outside ofthe can, is able generate sufficient heat in the surrounding can area toallow sealing of the lid 2 to the flange 4. In this way, this new methodof induction heating is able to focus the heating effect on the inwardlyprojecting flange 4 whilst keeping the exterior wall at a lowertemperature and thus preventing tin reflow and decoration degradation onthe exterior wall.

In a second embodiment, as illustrated in FIG. 4, the flange heatingsystem comprises two separately energised induction coils, an internalcoil 7 and an external coil 9, of which the internal coil 7 ispositioned coaxially within the external coil 9. A can 1 is raised andlowered with respect to the flange heating system, such that during theheating of the flange 4, the internal 7 and external 9 induction coilsare positioned adjacent to the flange 4, around the inner and outercircumference of the can 1 respectively. In this embodiment, theexternal induction coil 9 acts to heat the flange 4 via the externalwall, up to a temperature which is below that which would otherwisecause tin reflow and decoration degradation. The additional heatrequired to bring the flange 4 to the desired temperature is induced bythe internal induction coil 7. The electromagnetic fields from theexternal and internal induction coil overlap at the flange 4, causing acumulative heating effect. This particular embodiment is envisaged to beemployed in instances where it is necessary to heat the flange 4 at aparticularly fast rate.

In a third embodiment, as illustrated in FIG. 5, the flange heatingsystem comprises a single induction coil 10 with a set of “inner” turnsand a set of “outer” turns. When the flange heating system is applied toa can 1, the inner turns of the coil are positioned inside the can 1 andthe outer turns are positioned outside the can 1.

For all of the described embodiments, the design of the coils may beoptimised to achieve this directed heating. This may includeincorporating a copper plate 8 into the induction coil structure asshown in FIGS. 3, 4 and 5. As is known in the art, the coil may becooled by allowing water to flow through a passage extending through thecentre of the windings.

FIG. 6 illustrates schematically a production process for the heatingand sealing of metal cans 1, using a flange heating system of the typedescribed above (FIG. 3). The production process assumes that the canbodies 1 are open at both ends and that, after sealing of the foil lid2, the can is filled through the remaining open end, after which thatend is closed, e.g. with a seamable end. Of course, the process may beused to apply a foil lid 2 to an already filled can 1, provided thatthere is sufficient headspace to accommodate an induction coil withinthe can.

Considering the illustrated process further, the flange heating systemis mounted above a conveyor transporting cans 1 through the productionsystem, such that the coil extends downwards towards the conveyor. Eachmetal can 1 is held in place on a platform 12 which moves along theproduction line 11, raising and lowering the cans 1 appropriately. As itpasses beneath the flange heating system, a can 1 is raised so that theinduction coil is adjacent to the flange 4, and the coil energised bypassing an alternating current through it (the coil may be switched onand off or may be in a permanently on state). The metal can 1 is held ina fixed position relative to the flange heating system for the durationof the induction heating process. In order to maximise production speed,the sealing surface 5 of the flange 4 is expected to reach the requiredtemperature, for example 200° C., in the order of milliseconds. Once therequired temperature is achieved, the flange heating system is removedfrom the can 1 by lowering the platform 12 on which the can 1 is placed.The metal can 1 is then moved to the next section of the production line11, to a position beneath a lid holder 13. Each lid 2 is coated on thelower surface with an appropriate bonding material 3. The can 1 is againraised to a height at which the lower periphery of the lid 2 contactsthe sealing surface 5 of the flange 4. Pressure applied between thepeelable lid 2 and the sealing surface 5 of the flange 4, and theresidual heat within the sealing surface 5, will cause the lid 2 to sealonto the flange 4, with the bonding material 3 setting in the process.The platform 12 is then lowered to disengage the can 1 from the lidholder 13 and is moved to the next stage of the production line 11.

If multiple sealed peelable lids 2 are to be provided within a singlecan 1, the process outlined above may be repeated along the productionline 11.

It will be appreciated by the person of skill in the art that variousmodifications may be made to the above described embodiments withoutdeparting from the scope of the present invention. For example, in thecase where the lid material is itself able to adhere to the flange 4(e.g. where the lid is of a plastic or plastic coated material), theremay be no need to provide a separate layer of bonding material 3 betweenthe lid 2 and the flange 4.

The invention claimed is:
 1. A method of sealing a peelable lid to aninwardly projecting and circumferentially extending flange of a metalcan body, the method comprising: inserting an induction coil into thecan body, the induction coil including a first coil and a second coilthat each extend at least partially circumferentially about an interiorof the can body, wherein the step of inserting the induction coil intothe can body comprises: positioning the first coil above the flange, andpositioning the second coil below the flange; passing an alternatingcurrent through the induction coil to heat the flange; removing theinduction coil from the can body; and applying the peelable lid to theflange, whereby residual heat in the flange seals or aids the sealing ofthe peelable lid to the flange.
 2. The method according to claim 1,wherein the induction coil is an internal induction coil, the methodfurther comprising the step of locating an external induction coilaround the exterior of the can body and, substantially simultaneouslywith passing an alternating current through the induction coil, passingan alternating current through the external induction coil to applysupplementary heat to said flange.
 3. The method according to claim 2,wherein the internal induction coil and the external induction coil arecoupled together so that the same current passes through both coils. 4.The method according to claim 1, wherein a bonding material is presentbetween the peelable lid and the flange.
 5. The method according toclaim 4, further comprising applying a bonding material to a sealingsurface of the flange prior to applying the peelable lid to the flange.6. The method according to claim 4, wherein the bonding material ispresent on a sealing surface of the peelable lid.
 7. The methodaccording to claim 1, wherein the flange is located part way along thelength of the can body such that sealing of the peelable lid to theflange separates the can body into upper and lower compartments.
 8. Themethod according to claim 2, wherein the internal induction coil iscoaxial with the external induction coil.
 9. The method according toclaim 1, wherein the induction coil further includes a copper platepositioned between the first coil and the second coil, wherein the stepof inserting the induction coil into the can body further comprisespositioning the copper plate adjacent to the flange.